1. Field of the Invention
The present invention relates to a liquid crystal display device, and to a method of fabricating a liquid crystal display device, particularly a color filter on thin film transistor (COT) type liquid crystal display (LCD) device and a method of fabricating the COT type LCD device.
2. Discussion of the Related Art
In general, liquid crystal display (LCD) devices make use of optical anisotropy and polarization properties of liquid crystal molecules to produce images. When an electric field is applied to liquid crystal molecules, the liquid crystal molecules are rearranged. As a result, the transmittance of the liquid crystal molecules is changed according to the alignment direction of the rearranged liquid crystal molecules.
The LCD device includes two substrates disposed with their respective electrodes facing each other, and a liquid crystal layer is interposed between the respective electrodes. When a voltage is applied to the electrodes, an electric field is generated between the electrodes to modulate the light transmittance of the liquid crystal layer by rearranging liquid crystal molecules, thereby displaying images.
FIG. 1 is an exploded perspective view of a liquid crystal display device according to the related art. In FIG. 1, a liquid crystal display (LCD) device 11 includes an upper substrate 5, a lower substrate 22 and a liquid crystal layer 14 interposed between the upper and lower substrates 5 and 22. A black matrix 6 is formed on the upper substrate 5 and a color filter layer 8 including sub-color filters is formed on the black matrix 6. A common electrode 18 is formed on the color filter layer 8. A pixel electrode 17 and a thin film transistor (TFT) “T” as a switching element are formed on the lower substrate 22 in a pixel region “P.” The pixel electrode 17 is formed of a transparent conductive material such as indium-tin-oxide (ITO) and indium-zinc-oxide (IZO). The pixel region “P” is defined by a gate line 13 and a data line 15 and the TFT “T” disposed in matrix is connected to the gate line 13 and the data line 15. In addition, a storage capacitor “C” connected in parallel to the pixel electrode 17 is formed on the gate line 13. A portion of the gate line 13 is used as a first electrode of the storage capacitor “C”, and a metal pattern 30 with an island shape, which is the same layer and the same material as the source and drain electrodes of the TFT “T,” is used as a second electrode of the storage capacitor “C.” Because the metal pattern 30 is connected to the pixel electrode 17, the same signal is applied to the metal pattern 30 and the pixel electrode 17.
The upper substrate 5 and the lower substrate 22 may be referred to as a color filter substrate and an array substrate. Because the LCD device 11 is obtained by attaching the upper substrate 5 having the color filter layer 8 and the lower substrate layer 22 having array elements such as the gate line 13, the data line 15 and the TFT “T,” the LCD device 11 may deteriorate due to light leakage resulting from an alignment error. In order to overcome these problems, a color filter on TFT (COT) type has been suggested where a color filter layer is formed on the array substrate.
FIG. 2 is a schematic cross-sectional view of a COT type LCD device according to the related art. In FIG. 2, a COT type LCD device 40 includes a first substrate 50 having a TFT “T” and a color filter layer 68 in a pixel region “P” a second substrate 90 facing the first substrate 50, and a liquid crystal layer 80 between the first and second substrates 50 and 90. The first and second substrates 50 and 90 include a display region “DR” having the pixel region “P” and a non-display region “NDR” surrounding the display region “DR.” In the display region “DR,” the TFT “T” includes a gate electrode 52, an active layer 60, a source electrode 62 and a drain electrode 64. A gate line 54 and a data line (not shown) crossing each other to define the pixel region “P” are formed on the first substrate 50.
A color filter layer 68 including a red sub-color filter 68a, a green sub-color filter 68b and a blue sub-color filter (not shown) is formed on the TFT “T,” the gate line 54, and the data line (not shown). The color filter layer 68 corresponds to the pixel region “P.” In addition, a black matrix 71 and a shielding layer 70 are formed on the first substrate 50 including the color filter layer 68. The black matrix 71 is disposed in the non-display region “NDR” to thereby surround the display region “DR.” The shielding layer 70 is disposed between the sub-color filters 68a and 68b in the display region “DR.” The shielding layer 70 corresponds to a channel region “CH” of the TFT “T.” A pixel electrode 74 connected to the drain electrode 64 through a contact hole 77 is formed on the color filter layer 68 and a first orientation film 76 is formed on the pixel electrode 74 for an initial alignment of liquid crystal molecules. A second orientation film 94 and a common electrode 92 are sequentially formed on the second substrate 90.
FIGS. 3A to 3E are schematic cross-sectional views showing a fabrication process of a first substrate for a COT type LCD device according to the related art.
In FIG. 3A, a thin film transistor (TFT) “T” including a gate electrode 52, a gate insulating layer 57, an active layer 60, a source electrode 62, and a drain electrode 64 is formed on a first substrate 50 in a display region “DR.” The first substrate 50 includes the display region “DR” and a non-display region “NDR”. The active layer 60 includes an intrinsic amorphous silicon layer 60a and an impurity-doped amorphous silicon layer 60b. The active layer 60 under the source and drain electrodes 62 and 64 has a double-layered structure of the intrinsic amorphous silicon layer 60a and the impurity-doped amorphous silicon layer 60b, while the active layer 60 in a channel region “CH” between the source and drain electrodes 62 and 64 has a single-layered structure of only the intrinsic amorphous silicon layer 60a. In addition, a gate line 54 is formed on the first substrate 50 in the display region “DR.” A first passivation layer 66 is formed on the TFT “T.” The first passivation layer 66 may be extended to the non-display region “NDR.”.
In FIG. 3B, a color filter layer 68 including a red sub-color filter 68a, a green sub-color filter 68b, and a blue sub-color filter (not shown) is formed on the first passivation layer 66 in the display region “DR”. The color filter layer 68 is formed by repeating sequential processes of coating and patterning a color resist. Portions of the first passivation layer 66 corresponding to the channel region “CH” and the drain electrode 64 are exposed for a shielding layer and a pixel electrode in a subsequent step.
In FIG. 3C, a shielding layer 70 and a black matrix 71 are formed on the first substrate 50 including the color filter layer 68. The shielding layer 70 and the black matrix 71 are formed of an opaque material such as chromium (Cr) or an organic material including carbon (C). The shielding layer 70 is formed over the channel region “CH” to prevent external light from entering into the channel region “CH” of the TFT “T.” The black matrix 71 is formed in the non-display region “NDR” surrounding the display region “DR” to prevent light leakage at peripheral portions of the display region “DR.”.
In FIG. 3D, a second passivation layer 73 is formed on the shielding layer 70 and the black matrix 71. The second passivation layer 73 has a drain contact hole 77 exposing the drain electrode 64 through the first passivation layer 66.
In FIG. 3E, a pixel electrode 74 is formed on the second passivation layer 73 in the pixel region “P” by depositing and patterning a transparent conductive material. The pixel electrode 74 is connected to the drain electrode 64 through the drain contact hole 77.
Although not shown in figures, a common electrode is formed on a second substrate. After the first and second substrates are formed, a sealant is formed at a boundary portion of one of the first and second substrates and spacers are sprayed on one of the first and second substrates. Then, the first and second substrates are attached to form a liquid crystal panel and a liquid crystal material is injected into the liquid crystal panel, thereby completing the COT type LCD device.
In the COT-type LCD device, however, because a cell gap is fixed by ball spacers, uniformity of the sprayed ball spacers is poor and the ball spacers may conglomerate. Accordingly, display quality may deteriorate due to light leakage resulting from the non-uniformity of the ball spacers.